Battery charge determination, also known as “battery fuel gauging”, designates a process of determining the remaining charge in a battery. The information about a current state of charge of a battery may be displayed, for example during the use of a device functioning with the battery. This way, any user of the device may verify the remaining charge of the battery and take decisions regarding the use of the device accordingly.
Different ways of determining the current state of charge of a battery exist in the art. Some methods rely on voltage versus state of charge lookup tables. Such methods comprise measuring the voltage of the battery and determining the current state of charge as the state of charge associated to the measured voltage in the lookup table. Such methods may also comprise selecting the lookup table based on the temperature of the battery, which may be measured. However, these methods are not reliable in case the battery is in a charge or a discharge phase. Indeed, when the battery is in a charge or a discharge phase, the voltage varies in a non monotonic way with great amplitude (i.e. the voltage fluctuates a lot). Thus, there is no strong relationship between the voltage and the state of charge (in other words, different states of charge correspond to the measured voltage, rendering lookup tables unusable for determining an accurate state of charge).
For this reason, some methods, such as the ones disclosed in documents US 2011/057586 A1 and US 2007/096697 A1, determine the current state of charge of a battery relying on a shunt resistor in the battery path. The principle of these methods is to keep track of the charge provided by or to the battery. For a reliable determination, the value of the shunt resistor must be precisely known. This can either be done by using resistors with a low tolerance and thus a pre-known reliable value, or by calibrating the resistance.
Either way, the use of a shunt resistor makes the method costly. Another issue with thick-film shunt resistors is their temperature coefficient. Moreover, the presence of a shunt resistor creates a voltage drop in the circuit, which can be critical for nowadays devices. Indeed, there is a trade-off between the resistor value and the accuracy that is achievable. For accuracy reasons, the shunt resistance should be as high as possible. However, a high shunt resistance means potentially a high voltage drop, at least if the product has a need for high currents. Mobile phones and laptops are examples of products that require rather high peak currents. The obvious drawback with a shunt resistor is thus that an amount of power is dissipated in the shunt resistor. In particular, the voltage drop that is introduced can have a significant impact on the design in a device that is supplied from a single cell battery. Some of the emerging high capacity battery technologies have lower cell voltage, which might introduce the need for a boost circuit that normally consumes the additional capacity gained.
This is the case for LDO. Every mV counts when defining cut-off voltages and headroom for LDO. Indeed, when a complete system is defined, i.e. during the systemization phase of a product, it is preferred to make sure that all sub-systems can be supplied with power. Normally, a wide range of LDO's (Low-dropout regulator) and buck converters are used. A linear, low-dropout regulator (LDO) can have a dropout voltage in the range of 40-100 mV (i.e, the voltage on the supply node typically needs to be at least 40-100 mV higher than the output voltage). The consumer that requires the highest voltage may define where the cut-off voltage is placed. The cut-off voltage is the voltage where an automatic shutdown of the device is triggered due to that the systems within the device can no longer be supplied properly. In a system supplied from a low-voltage battery, every mV additional voltage drop will move the cut-off voltage higher up in the V/SOC curve, meaning that the complete battery capacity is not utilized.
Also, when a shunt resistor is used, the shunt resistor is often placed on the negative electrode (anode). This makes the ground plane less perfect. This can raise issues, as the product's ground plane is not truly zero resistance towards the battery.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.